Retractors are commonly used in surgical procedures to separate and expand an incision to access the surgical site and to minimize trauma to the patient. While there are many styles, shapes and sizes of retractors, the typical retractor used in spinal surgery comprises a plurality of retractable blades, which may include two to four or more blades that are introduced through the surgical incision to form a protected corridor to the surgical site. Various mechanisms are provided to move one or more blades in different directions so as to expand the incision and to hold the blades in the expanded position. One factor in the surgeon's decision as to the type of retractor used is the control of the blade movement. Blades are often configured to not only expand outwardly so as to expand the corridor but also to pivot or toe at their distal ends so as to increase the opening of the corridor adjacent the surgical site. In addition, the size of the retractor is often of consequence, with the surgeon typically seeking to minimize the overall footprint of the retractor for ease of handling, placement and use during surgery.
One form of a retractor that has emerged primarily in minimally invasive spine surgery is a screw-based retractor that combines the functions of both a retractor and a distractor/compressor. In such a screw-based retractor, a pair of retractor blades may be attached to the patient's anatomy by connecting each blade to a pedicle screw that is anchored to respective vertebra of the patient. Moving the blades relatively away from each other not only provides retraction of surrounding soft tissue, but also distracts the respective vertebra for subsequent fixation. Compression may be effected by moving the blades relatively toward each other. Examples of such screw-based retractors are described in U.S. Pat. No. 9,216,016, entitled “Surgical Device for Minimally Invasive Spinal Fusion and Surgical System Comprising the Same”, issued to Fiechter et al. on Dec. 22, 2015, and U.S. Pat. No. 9,414,828, entitled “Integrated Retractor-Distractor System for Use with Modular Bone Screws”, issued to Abidin et al. on Aug. 16, 2016. While such screw-based retractor systems exhibit certain improved features, a screw-based retractor having further degrees of freedom for enhanced user applicability is desirable.